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Tire Size/ Speedometer Question

Discussion in 'Car Forum' started by TxTech, May 28, 2008.

  1. TxTech


    Jun 3, 2007
    Heath/Lubbock, TX
    On my Grand Cherokee I went from 235/65/17 to 245/75/17. A tire calculator I looked at said when my speedometer says 60MPH I am actually going 65MPH. Does this change throughout the speed range or am I always 5MPH off?
  2. It will change throughout the speed range. The faster you go, the bigger the difference will be.

  3. So when the car isn't moving, you're actually going 5mph... :upeyes:
  4. Bowtie

    Bowtie NRA MEMBER

    Dec 18, 2004
    Why is that? Do the tires get bigger as he goes faster?
  5. If he's off by 5% of 50 MPH that's 2.5 MPH - at 100 MPH the variance would be 5 MPH. Yeah the added circumfrence will make the speedo appear slower than actual. measure the tires though - tire manufacturers are like shoes and jeans - actual measurements may vary - you may be better or worse off than you think you are - depending on the real measurements.

    Why did you up the sidewall? Add ground clearance??